This invention relates to a packet communication system used for packet mode communication of a signal, such as, an audio signal or a video signal through a communication network, and more particularly to a packet communication system suitable for packet mode communication of a signal, such as, a speech signal between communication networks using signal types different from each other.
With the recent progress of the architecture of a LAN (local area network), the network system connecting small size computers, such as, personal computers using the LAN is also increasing. Thus, an attempt is now being made for integration of communications by permitting voice communication on the LAN in addition to a conventional telephone network (including a PBX network that is a private branch exchange network).
As an example of such an attempt, there is a “voice communication system” as shown in FIG. 4. This voice communication system is composed of telephones connected to a conventional telephone network, packet communication units connected to a packeted speech signal communication network (referred to hereinafter as a packet communication network), and a communication gateway used to realize mutual connections between the telephones and the packet communication units. A network carrying such a speech packet on the LAN described above is an example of the packet communication network.
Referring to FIG. 4, the reference numerals 21 and 22 designate telephones connected to a private branch exchange network (a PBX network) 13 and a public telephone network 20, 104 designates a subscriber line of the private branch exchange network 13, 105 designates a subscriber line of the public telephone network 20, and they have the same interface. On the other hand, the reference numeral 101 designates a packeted speech signal transmission network in the form of, for example, a LAN based on the CSMA/CD (carrier sense multiple access with collision detection) scheme according to the standards defined in the IEEE 802. 3. A plurality of packet communication units 14, 15 and 16 are connected to this LAN 101.
The packet communication units 14, 15 and 16 are in the form of small size computers, such as, personal computers provided with handsets 17, 18 and 19 respectively. These packet communication units 14, 15 and 16 have, on one hand, functions of packeting speech signal inputs from the handsets 17 to 19 and sending out the packeted speech signals onto the LAN 101 and, on the other hand, functions of converting the packeted speech signals received via the LAN 101 into ordinary voice streams again and reproducing the original speech by the handsets 17, 18 and 19.
The reference numeral 11 designates a mail box connected to the LAN 101 to store a filed voice mail. The packet communication units 14, 15 and 16 can store such filed voice mails in the mail box 11 through the LAN 101 and can also fetch such voice mails, transmitted from the other packet communication units and stored in the mail box 11, so as to internally convert the stored voice mails into voice streams again and reproduce the original speech signals by the handsets 17, 18 and 19.
The reference numeral 12 designates a communication gateway which controls communication between the LAN 101 that is the packet communication network and the private branch exchange network 13 that is the conventional telephone network. In this case, a subscriber line accommodating an existing telephone is commonly used as a connection line 102 between the private branch exchange network 13 and the communication gateway 12. However, this connection line 102 may be an interface between the telephones in the private branch exchange network.
Suppose now the case where voice communication is made among the packet communication units 14, 15 and 16 in the LAN 101 in the voice communication system having the structure described above. In the case of making voice communication between, for example, the packet communication units 14 and 16, the speech packet is transferred through a communication path 121. On the other hand, in the case of making communication between the packet communication unit 14 and the private branch exchange network 13 or the telephone 21 or 22 connected to the public telephone network 20, the speech packet is transmitted through a communication path 122 in the LAN 101. In this case, the packeted speech signal on the LAN 101 is converted by the communication gateway 12 into the corresponding voice stream before being transferred to the private branch exchange network 13 or the public telephone network 20. In the case of a speech signal transmitted from the side of the private branch exchange network 13 or the public telephone network 20 toward the packet communication unit 14 in the LAN 101, its voice stream is packeted by the communication gateway 12 to be sent out to the LAN 101.
In the case of integration of voice communication in a packet communication network in the form including a conventional telephone network, it is an undeniable fact that the number of users having their telephones connected to the conventional telephone network is actually predominantly large although the number of terminals connected to the packet communication network is increasing. Therefore, when the desired integration of the voice communication is considered, the amount of communication between the telephones connected to the conventional telephone network and the terminals connected to the packet communication network is expected to considerably increase.
On the other hand, when the future of the packet communication network is reviewed, an undeniable situation will appear in which the users possessing their telephones already become new members of the packet communication network. For example, although specific users, such as, persons holding administrative positions in a company only possess the packet communication units in the LAN in the company at present, it is apparent that there will be an increasing tendency of introducing the packet communication units in future so that all the members of the company can possess the packet communication units.
However, the prior art packet communication units connected in the packet communication network are basically considered to make communication in packet form. Thus, the desired integration of the voice communication in the form including the conventional telephone network will bring forth such a problem that the communication gateway tends to be overloaded due to concentration of loads, resulting in impossibility of connection or communication.
Further, when the packet communication unit is newly provided for the user possessing his telephone already, the same user possesses both the telephone and the packet communication unit. As a result, the cost inevitably increases, and such a problem arises that the user cannot deal with simultaneous occurrence of a communication request from both the telephone and the packet communication unit. The same applies also to the case where the telephone is newly provided for the user who possesses the packet communication unit only.